For medium- and/or high-voltage uses, i.e. in general terms for voltages which are greater than 1 kV, the high voltages mean that switching devices of greater complexity are required which can withstand the electric fields which occur, are as resistant as possible to degradation effects and are also intended to avoid jump occurrences outside the actual switching chamber.
A classic example in this regard are vacuum circuit breakers (VCB) which are core components in energy transmission and distribution, in particular in switching systems thereof. They cover a large portion of the medium-voltage switching uses, i.e. switching uses within the range of 1 kV to 52 kV, for example, and also a relevant portion in low-voltage systems. The use thereof in high-voltage transmission systems, therefore, for example, for voltages of greater than 52 kV, is also increasing. While a VCB is closed for most of the time and consequently provides contact-making of the conductor elements, its primary task is the interruption of currents in alternating current systems under rated conditions, therefore in particular for switching on and switching off rated currents, or else preferably for interrupting currents under fault conditions, in particular in order to interrupt short circuits and to protect the systems. Other uses include the pure switching of load currents using contact-making conductor elements, which switching is generally used in low- and medium-voltage systems.
The vacuum interrupter (VI) is the core element of a VCB. A vacuum interrupter generally has a pair of contacts which are formed by corresponding conductor elements, of which at least one can be moved by means of a moving mechanism in order to be able to bring about the open and closed states of the switching device. One conductor element here is customarily moved axially with respect to the other fixed conductor element. The contacts can be manufactured on current-conducting bolts which are composed in particular of metal which provide both current conduction and heat conduction, and also the mechanical means for holding and/or moving the contacts.
A VI furthermore comprises a vacuum-tight housing and the moving mechanism mentioned and can also comprise a metal bellows which is connected on one side to the housing and on the other side to the moving conductor element, in particular the moving bolt. The housing is substantially formed by an insulating component, i.e. an insulator, for example a ceramic tube which is connected to the conductor elements by connecting elements, wherein, for example, use can be used of metal caps or the like which seal the insulating component in the axial direction in order to form the switching chamber. Within the switching chamber prevails a permanent high vacuum of less than 10−8 Pa which can be ensured, for example, for operating periods of at least 30 years by means of an appropriate configuration of the housing and of the caps. The vacuum is necessary in order to ensure the “make-brake operations” and to ensure the insulating properties of the switching device in the open state.
When the switching device is in an open state, firstly the rated voltage of the system has to be insulated, but, secondly also surge voltages of high amplitudes which may be triggered, for example, by a lightning strike on the system. If the switching device transfers from the closed into the open state, as consequently the contacts with the conductor elements are placed at a distance from one another, it is necessary to interrupt rated currents or short circuit currents which lead to the emergence of temporary voltage peaks via the VI that are significantly higher than the rated alternating voltages of the system.
High voltages in vacuum systems customarily produce free electrons due to field emission processes if the electric field strength is sufficiently high. The acceleration of the electrons in the high electric fields increases the kinetic energy of said electrons, for example up to energies which exceed some tens or even hundreds of KeVs. The interaction of said highly energetic electrons with the housing structures leads to the production of highly energetic X-ray radiation which can leave the vacuum interrupter. Whereas, under customary conditions, the fault current within the vacuum interrupter is minimal and does not produce any significant X-ray radiation portions, circumstances may arise, for example if temporary voltage peaks of high amplitude occur, in which the X-ray radiation which arises produces free electrons on and/or in the vicinity of the outer surface of the insulator. Said electrons can be accelerated by the electric fields on the insulator surface and in the vicinity thereof, interfere with the electric field distribution in sensitive regions and lead to a gap flashover, which leads to a fault in the operation of the vacuum interrupter.
Even in situations in which no detectable X-ray radiation exists, for example in low- and medium-voltage uses, the high electric fields in critical regions of the vacuum interrupter, for example at the connection of the insulator and of the metal caps by soldering (brazing), may lead to the emission of electrons, which leads to a significant amount of field emission. These electrons can also interfere locally with the electric field and lead to further strengthening of the field and/or to charge multiplication due to electron avalanches which, in turn, may result in the loss of the insulating strength and/or in the voltage resistance of the vacuum interrupter.
There are similar challenges on the inner surface of the vacuum interrupter, while an additional problem has to be solved. The interruption in the current (rated current and also short circuit current) causes parts of the contact material to evaporate and be distributed as hot metal vapor within the switching chamber. Said metal vapor can be deposited on the insulator surface and builds up a conductive metal layer over time. Said metal layer, even if it is only weakly conductive, can likewise interfere with the electric field outside and within the vacuum interrupter and can consequently cause a deterioration in the voltage resistance capability of the vacuum interrupter over time. Although it has been proposed in this context to provide, in the contact-making region of the conductor elements, a shielding element, which is likewise composed of metal, for catching free metal particles of the conductor elements, said shielding element, however, also influences the field distribution within the switching chamber, but also on the insulator.
For the reasons mentioned, the insulator, which is generally realized from ceramic, has to be capable of withstanding high voltages over its surface, even if X-ray radiation and free electrons are present or, in some cases, even if the insulator is soiled by dust particles which are accumulated electrostatically on the outer surface of the insulator. Since the insulator contributes significantly to the costs of a vacuum interrupter (or other switching devices) and also has a negative effect on the costs of other structural elements of the vacuum interrupter (or other switching devices), the insulator has to be optimized in respect of maximum dielectric strength for a minimal size.
This problem has been solved up to now in that the inner and the outer geometry of the vacuum interrupter has been selected in such a manner that the anticipated electric field strengths do not exceed empirically derived limits for a certain geometry of the vacuum interrupter. Since said limits cannot be precisely predicted, in particular for triple point regions and sharp metal edges, the design of vacuum interrupters depends not only on calculations regarding the electric field during the development process, but also requires a great deal of empirical optimization. This also refers to buildup of metal layers from the inner surfaces of the insulator, which layers, as already mentioned, are customarily intended to be avoided nowadays by using shielding structures (shielding elements) within the switching chamber. Nevertheless, the depositions of the metal vapor and the effect thereof on the dielectric strength of the vacuum interrupter cannot today be quantitatively predicted in a sufficiently precise manner.
Furthermore, it should be noted that the design processes mentioned all lead to a reduction in the insulating properties of the outer structure of the vacuum interrupter significantly under the dielectric strength of air or other gases which surround the vacuum interrupter, and therefore insulator sizes (length, diameter) which are not optimum in respect of costs and construction space are required. The addition of shielding elements with respect to the metal vapors leads to distortions of the electric fields, which occur during the operation, at the insulator, which may lead to strong fields at certain points and consequently to an overloading of the insulator caused by charges building up there. As has already been explained, other causes can also lead to such local high fields at the insulator of the housing of the vacuum interrupter, wherein the problems explained here also apply to other switching devices in addition to the vacuum interrupter, which is mentioned by way of example.